AGC fine-tuning be the adaptive time domain equalizer

ABSTRACT

A method of improvement for gain control for a Quadrature Amplitude Modulation (QAM) signal within a communication system with at least one digital modem. The communication system utilizes a non-decision aided AGC at the intermediate frequency level and a modified Adaptive Time Domain Equalizer (ADTE) to fine tune the level of baseband signals, perform phase and amplitude correction, and channel equalization. The center tap coeficients of or dynamically modified by the ADTE. The resynchronization time and phase and amplitude imbalance correction will be reduced as a consequence of the controlling of the signal gain by a non-decision aided automatic gain controller.

BACKGROUND OF THE INVENTION

[0001] One of the main problems in digital wireless communication system is the dispersive fading cause by multi-path propagation in transmission channels. Therefore high performance adaptive equalizers play a very important role in modern digital wireless communication systems to combat multi-path fading. Inter symbol interference is one effect of multi-path fading.

[0002] In a digital modem, the gain control signal can be derived directly from the received I F signal. This approach however lacks precision, especially in a selective fading environment. To improve the accuracy, Automatic Gain Control circuits are built based on decision-directed algorithms. Thus in order to have a valid and stable control for the Automatic Gain Control AGC circuit, it is required to wait for the synchronization loops (carrier/clock) and also the equalizer to stabilize. This process takes several symbol intervals before all the loops converge and increases the re-synch time.

[0003] Standard implementations of Adaptive Time Domain Equalization (ATDE), requires in order to eliminate inter symbol interference (ISI) only to adjust the side coefficients of the ATDE relative to the center tap coefficients. The center coefficients for the through- and cross-sections of the ATDE are normally set to 1 and 0, respectively. These coefficients are fixed and are not dynamic.

[0004] By using the present invention, an intermediate frequency (IF) automatic gain control can be used alone with the incorporated AGC in the ATDE circuit. This implementation has the advantage of reducing the re-synch time for the modem as well as capability of correcting I/Q amplitude and phase imbalance.

SUMMARY OF THE INVENTION

[0005] An object of the invention is an improvement for a method and apparatus for gain control. The method of gain control for a QAM signal in a communication system includes at least one digital modem. The improvement involving using a non-decision aided algorithm at the intermediate frequency level for coarse tuning and using a modified Adaptive Time Domain Equalizer (ATDE) to fine tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with a channel equalization.

[0006] Another object of the invention is a method of reducing re-synch time and I/Q amplitude and phase imbalance correction in a digital modem for a QAM signal. The method involving controlling the signal gain with a non-decision aided automatic gain controller to produce a baseband signal, and tuning the baseband signal in an Adaptive time domain equalizer utilizing non-fixed center tap coefficients.

[0007] Yet another object of the invention is an improvement for a method, in a communication system including at least one digital modem, for gain control for a QAM signal. The improvement comprising using a modified Adaptive Time Domain Equalizer (ATDE) to tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with channel equalization, wherein center tap coefficients are not fixed.

[0008] These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is an embodiment of gain control with AGC and ATDE according to the present invention.

[0010]FIG. 2 is a representation of ATDE with dynamic center taps coefficient updating according to the present invention.

[0011]FIG. 3 is a QAM constellation with input and output symbols by the present invention

DETAIL DESCRIPTION

[0012]FIG. 1 is a representation of an embodiment of the invention. The automatic gain control 101 is a non-decision aided gain control that is used on the intermediate frequency signal for course gain control. Simple techniques that are known in the art can be employed by the AGC 101 such as peak detection or envelope detection. The AGC 101 supplies a gain control signal to the amplifier 103, which in turn amplifies the signal. The modem 102 operates using decision directed algorithms. Carrier recovery, timing recovery and base band AGC can be all controlled by using decision from the output of a modified ATDE system.

[0013] The modified ATDE system unlike prior solutions uses the center tap coefficients of the equalizer to fine tune the level of the baseboard signal before delivering it to the decision stage. The center tap coefficients are not fixed, rather they are dynamic and are updated as the equalization algorithm updates other coefficients. In order to protect the ATDE from divergence, an expected dynamic range for the fine tuning, the center coefficients are limited to a maximum and minimum value. These maximum and minimum values are predetermined and adjustable by the user. These values can be percentage deviations from the expected value, for example + or −5%. Because of the course tuning by the non-decision directed AGC, the range of center tap coefficients can be limited.

[0014] In FIG. 2, a portion of the ATDE 200 for dynamic modification of the center tap coefficients prior to the decision stage is shown. The coefficients update algorithm 201 updates the center tap coefficients in the same manner as the side coefficients. The type and methods of these algorithms are well known and are discussed in detail in Reeve, H. C. “Transversal Equalizer for Digital Radios” Globecom 1989, Conference Proceedings, Vol. 1 11.7.1-11.7.5, Nov. 27-30, 1989 and Yom et al. “A 55 Mbaud Single Chip Complex Adaptive Transversal Equalizer for Digital Wireless Communications Systems” Proceedings of the IEEE, 1997, pp.151-154, both of which are hereby incorporated herein by reference. The coefficients can be updated every symbol period or fractions of symbol periods. The output of the coefficient update algorithm 201 is supplied to a low pass filter 202 for each of the four taps. The low pass filter 202 or accumulator as use in digital systems operates on the coefficients and passes them to a limiter 203. The limiter 203 regulates the signal to prevent divergence with the user determined maximum and minimum values for the center tap coefficients. In extreme cases, the coefficients saturate to the maximum or minimum values. The resultant center tap coefficients are then applied to the center tap signal in multiplier 204.

[0015] The respective output of the tuned I/Q signals are cross added as is known in the art at adders 205, resulting in I′ and Q′ base band signals which proceed to the decision stage.

[0016] In FIG. 3, the results of simulated +/−2 dB deviation in the I/Q signal corrected by the output the modified ATDE are shown. The Quadrature Amplitude Modulation (QAM) constellation with a +2 dB change in the base band level is represented by “+”, whereas the constellation with a −2 dB change in the base band level is represent by “*”. The output of the modified ATDE is shown as “o”. In both cases the ATDE was able to fine-tune the I/Q signals.

[0017] The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. The present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 

We claim:
 1. In a communication system including at least one digital modem, a method of gain control for a QAM signal, the improvement comprising the steps of using a non-decision aided algorithm at the intermediate frequency level for coarse tuning and using a modified Adaptive Time Domain Equalizer (ATDE) to fine tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with a channel equalization.
 2. The method of claim 1, wherein the fine-tuning uses center tap coefficients.
 3. The method of claim 2, wherein the center tap coefficients are dynamic.
 4. The method of claim 3, wherein the center tap coefficients are updated at least each symbol period.
 5. The method of claim 3, wherein the center tap coefficients are limited to a predetermined range
 6. The method of claim 1, wherein the ATDE is internal to the digital modem.
 7. The method of claim 1, wherein the coarse AGC can use peak detection.
 8. The method of claim 1, wherein the communication system is a cable network system.
 9. The method of claim 1, wherein the communication system is a point-to-point communication system.
 10. The method of claim 1 wherein said communication system is a point-to-multipoint communication system.
 11. The method of claim 10 wherein said communication system operates in the millimeter wave frequency spectrum.
 12. The method of claim 11 wherein said communication system is a time division multiple access system for communicating data in a frame format.
 13. The method of claim 12 wherein the data density within each frame is dynamically variable.
 14. The method of claim 12 wherein said communication system is a time-division duplex system.
 15. The method of claim 14 wherein said communication system is an adaptive time division duplex system.
 16. The method of claim 15 wherein the forward/reverse ratio is dynamically configurable.
 17. A communication system with at least one digital modem for processing QAM signals, the improvement wherein the at least one digital modem comprises; an automatic gain controller for coarse tuning of the baseband signal and an ATDE for fine tuning the baseband signal
 18. The communication system of claim 17, wherein the ATDE modifies a center tap coefficient within a predetermined range
 19. The communication system of claim 18, wherein the ATDE includes a lowpass filter/accumulator and a limiter.
 20. The communication system of claim 17, wherein the automatic gain controller uses peak detection.
 21. The communication system of claim 17, wherein the communication system is a cable network system.
 22. The communication system of claim 17, wherein the communication system is a point-to-point communication system.
 23. The computer system of claim 17 wherein said communication system is a point-to-multipoint communication system.
 24. The computer system of claim 23 wherein said communication system operates in the millimeter wave frequency spectrum.
 25. The computer system of claim 24 wherein said communication system is a time division multiple access system for communicating data in a frame format.
 26. The computer system of claim 25 wherein the data density within each frame is dynamically variable.
 27. The computer system of claim 25 wherein said communication system is a time-division duplex system.
 28. The computer system of claim 27 wherein said communication system is an adaptive time division duplex system.
 29. The computer system of claim 28 wherein the forward/reverse ratio is dynamically configurable.
 30. A method of reducing re-synch time and I/Q amplitude and phase imbalance correction in a digital modem for a QAM signal comprising the steps of controlling the signal gain with a non-decision aided automatic gain controller to produce a baseband signal, and tuning the baseband signal in an Adaptive time domain equalizer utilizing non-fixed center tap coefficients.
 31. The method of claim 30, wherein the center tap coefficients are dynamic.
 32. The method of claim 31, wherein the center tap coefficients are limited to a predetermined range.
 33. The method of claim 32, wherein the predetermined range is user configurable.
 34. In a communication system including at least one digital modem, a method of gain control for a QAM signal, the improvement comprising using a modified Adaptive Time Domain Equalizer (ATDE) to tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with channel equalization, wherein center tap coefficients are not fixed.
 35. The method of claim 2, wherein the center tap coefficients are dynamic. 